Thau lagoon is a Mediterranean shellfish ecosystem with large biomasses of oysters growing in waters with high residence time due to low tidal ranges. The influence of filter feeders (oysters and their epibiota) on the spatial distribution of particulate and dissolved compounds in the water column of Thau lagoon was studied through its variation with time. In 1991/1992, daily variations were investigated in pens, corridors and outside shellfish farming zones for nutrients, chlorophyll a and primary production. Salinity, dissolved oxygen, nutrients, organic matter and chlorophyll a were also monitored in surface waters inside and outside shellfish farming zones each week from January 1993 to March 1994. The presence of shellfish farms led to a decrease by only a few percent of oxygen concentrations in their vicinity, but the mean (± SE) deficits of chlorophyll a and POC concentrations were 44 ± 4% and 26 ± 9% respectively in the eastern zone (8 m). The shift induced by filter feeders in phytoplankton composition favoured picophytoplankton with higher growth rates. But the summer increase in phytoplankton growth rate was stronger than the positive feedback due to filter feeder filtration. Summer was determinant for the growth of oysters owing to enhanced regenerated primary production. During this period, filter feeders were not food limited, while they tended to control phytoplankton biomasses and production the rest of the year. The nutrient excess in shellfish farming zones was highly significant, with increases of 73 ± 16, 36 ± 12 and 19 ± 8% for ammonia, phosphates and silicate respectively in the eastern zone. In the western zone, the nutrient excess was less strong by half for ammonia and phosphate, because the lower depth (4 m) allows light to reach the bottom and enables benthic macroflora to grow on nutrients of benthic origin. The decline of phytoplankton biomasses in shellfish farms induced a decrease in the nutrient demand, especially for ammonia. This situation was likely to favour nitrification, which led during autumn to higher nitrate concentrations within shellfish farming zones than outside. Therefore, filter feeders were able to alter the dominant biogeochemical process in the water column by stimulating nitrification. KEY WORDS: Suspension-feeding bivalves · Phytoplankton · Nutrient cycling · Carrying capacity · Thau lagoonResale or republication not permitted without written consent of the publisher

Eucaryotic picophytoplankton (PEUC), picocyanobacteria, and larger phytoplanktonic cells from the Thau Lagoon (northwest Mediterranean coast) were numbered by flow cytometry from November 1991 to February 1994. PEUC cells dominated the phytoplanktonic assemblage and exhibited seasonal dynamics. Monthly mean abundances of larger phytoplanktonic cell and PEUC were significantly correlated with temperature and solar irradiance values, with the highest correlations having been obtained for PEUC abundances. The mean abundance of total picophytoplankton (3.5 × 104 cells ml‒1) was among the highest recorded in marine waters. The dominance of picophytoplankton over larger phytoplanktonic cells (average abundance, 5 × 103 cells ml‒1) in the nutrient‐rich waters of the lagoon was related to grazing from large‐scale shellfish breeding (oyster’s ≈35,000 t), which seems to act preferentially on the largest cells. Several hypotheses, including the potential selective effect of copper, were proposed to explain the dominance of the PEUC form (average abundance, 3.4 × 104 cells ml‒1).

Physical and chemical characteristics (temperature, salinity, dissolved oxygen), nutrients [dissolved inorganic nitrogen (DIN), soluble reactive phosphorus (SRP) and silicate], dissolved organic nitrogen (DON) and phosphorus and particulate matter [particulate organic carbon (POC) and nitrogen (PON) and chlorophyll a] were measured at a station located in the deepest part of the Thau lagoon (8.5 m), France, during a 10 d period of bottom anoxia in summer 1994. The upper 8 cm of sediment were also analyzed for ammonium (NH,') and SRP concentrations in the pore.water. The study period was characterized by mean wlnd speed under 4 m S-' which induced an increase of surface temperature from 18 to 2g°C with the formation of a thermocline. The correlation (p < 10-4) between the wind speed averaged over the previous day and the difference between bottom and surface temperatures showed that the wind conshtuted the main vector of vertical mixing. The lack of wind led to dissolved oxygen depletion in the bottom 2 m and to a strong porosity increase in the upper 10 cm of sediment (80 to >95%). Anoxic conditions increased NH,+ and SRP concentrations in porewater from 231 + 89 to 1305 + 305 (+ SD) ).]M and from 6.6 + 0.8 to 108 t 43 PM respectively. The potential increase of NH,' concentrations in porewater estimated from the mineralization of the microphytobenthos explained 30% of the measured increase in the upper 8 cm of porewater. The study period was characterized by an increase in NH4+, SRP and dissolved Si concentratlons in the bottom water column (maxima respectively 24.2, 4.9 and 57 PM). Linear regressions between nutrient concentrations in the water column and temperature revealed a strong enhancement of bottom fluxes during anoxia. Nitrate + nitrite (No3-+ NO2-) were absent in anoxic waters and remained below 0.5 pM in oxic waters. Monthly concentrations of SRP in the water column of the Thau lagoon from 1970 to 1994 revealed anoxia events by summer peak values corresponding to strong bottom anoxia. Nevertheless, the general trend was a decrease due to the gradual control of eutrophication which should lead to the disappearance of bottom anoxia within the next decade. A fraction of the nutrients of benthic origin was transformed into planktonic particulate matter vla prlmary production whlch increased the concentrations of chl a from about 1 to 15 pg I-'. Concentrations of DON and PON in the water column were significantly correlated, suggest~ng that DON compounds were released by the pelagic food web but not directly by the sediment. The increase of total nitrogen (DIN + DON + PON), considering the water column as a closed box during 10 d of winds under 5 m S-', was estimated at 1.7 pm01 N 1-' d-' and would correspond to a benthic flux of 600 pm01 N m-2 h-' for 10 d.

Nitrate uptake kinetics of different phytoplankton communities exhibit great variability over a 1-year period. Classical saturation kinetics accompanied by an induction phase are shown by blooms of Chaetoceros sp. in low nitrate waters. Biphasic kinetics, with transition points between 10 and 50 M, are shown by blooms of Skeletonema costatum and flagellates, while unsaturated kinetics (up to 100 M) are shown by Thalassiosira blooms and flagellate blooms. The latter also exhibit surge uptake of nitrate in situations of negative growth rates and rather high ammonium levels. The uptake patterns of the three diatom genera are similar to those observed in cultures or consistent with known internal nitrate accumulation characteristics. Based on the kinetic patterns presented, it is apparent that, for nitrate concentrations near 50 M such as in upwelling areas, nitrate uptake will be underestimated in present models of nitrate assimilation by a factor of almost 2 for S. costatum and a factor of about 3 for Thalassiosira sp. Academic Press Limited